The field of the invention is that of optical link transmission networks. The invention relates more particularly to optical networks suitable for relatively small geographical areas, such as access or metropolitan networks.
An optical network generally comprises a plurality of stations able to send and receive optical signals to and from other stations of the network. These exchanges of information are effected by means of optical links to which access nodes that serve the respective stations are connected.
The wavelength division multiplexing (WDM) technique is advantageously used to exploit the bandwidth capacity of the optical links.
Moreover, if the network is sufficiently small, systems for regenerating individual channels need not be provided. A network of this kind is referred to as a “transparent” network, but can nevertheless include optical amplifiers for amplifying all the channels of the transmitted WDM multiplexes simultaneously. If the network additionally comprises no line amplifiers, it is referred to as a “passive” network.
The invention applies to a network of any of the types referred to above if the network has a tree configuration. To simplify the disclosure, FIG. 1 shows diagrammatically the simplest embodiment of a network of the above kind, i.e. a passive transparent network using a single wavelength in each signal propagation direction.
In this example, the network comprises three stations ST1-ST3 each of which is adapted to send to a concentrator 1, sometimes referred to as a hub or optical line terminal (OLT), via an optical interconnection tree that connects respective sending points K1-K3 of the stations to the same receiving point I of the concentrator. The tree is made up of optical link comprising dedicated access links F1-F3 connecting the sending points K1-K3 of the stations to respective inputs of a coupler C and a common optical link OL having a first end coupled to the receiving point I and a second end J connected to an output of the coupler C.
In their most economical embodiment, the access links F1-F3 and the common optical link OL are simply fibers, but links provided with amplifiers may be more appropriate for longer distances.
The property of the links of being able to propagate signals in two opposite propagation directions (the “uplink” direction from the stations to the concentrator and the “downlink” direction from the concentrator to the stations) may be exploited to exchange signals between the concentrator and the stations. In this case, the sending points K1-K3 and the receiving point I also constitute receiving points and a sending point, respectively. A network of the above type is described below, although the invention may also be applied to networks in which uplink and downlink traffic are carried by separate links.
To communicate with the stations, the concentrator includes an optical sender and an optical receiver (not shown), each of which is coupled to the common optical link OL at the point I. The concentrator generally also enables the access network to communicate with other access networks and/or with an external interconnection network (not shown).
Similarly, each station ST1-ST3 includes an optical sender TX and an optical receiver RX coupled to the associated fiber F1-F3 via a corresponding access node AN1-AN3, an external port of which constitutes one of the sending (and receiving) points K1-K3.
In the simplest case, the concentrator and the stations each comprise one sender and one receiver. The sender of each station then uses a carrier wave having a first or “uplink” wavelength λu common to all the stations, this wave propagating in the direction of the concentrator, which therefore includes a receiver able to detect the uplink wavelength λu. Similarly, the sender of the concentrator uses a carrier wave having a second or “downlink” wavelength λd, this wave propagating in the direction of the stations. Each station therefore includes a receiver adapted to detect the downlink wavelength λd.
In practice, the signals exchanged in a network of the above kind consist of packets, for example Ethernet frames or ATM cells. This applies in particular to an Ethernet passive optical network (EPON) and an ATM passive optical network (APON).
As shown in FIG. 1, by virtue of the coupler C, the respective uplink signals S1, S2, S3 sent by the stations ST1, ST2, ST3 converge at the end J of the common optical link OL. Accordingly, with the uplink wavelength λu being common to the stations, it is necessary to take into account the risk of collisions between the uplink signals, i.e. the fact that a plurality of signals from a plurality of stations may reach the end J simultaneously.
A first solution to this problem is for the stations to use the time division multiple access (TDMA) technique to send signals. Thus the stations are allocated respective separate successive time windows and each station can send only during its own time windows, which thus define a TDM time channel.
This method presupposes that the sending phases of the respective stations are well synchronized. This necessitates accurate synchronization means common to all the stations of the network. Because the stations are far apart, implementation by the usual electrical means is relatively difficult and costly.
Moreover, if the simpler solution of providing predefined time windows is adopted, changes in the respective bandwidth requirements of the stations are totally ignored. This generally results in sub-optimum use of the resources of the network.
It is nevertheless possible to envisage modifying the addresses of the windows assigned to the respective stations dynamically, the modifications taking into account changes in the instantaneous requirements of the respective stations. This implies centralized management of the network which, according to the conventional approach, requires a network controller and electrical means for exchange of information between each station and the controller. This is shown in FIG. 1 by the presence of the controller 2 and control links symbolized by dashed-line arrows.
This solution is even more costly than the preceding solution, since it requires each station to be provided with suitable control means such as dedicated dialogue and processing circuits.
Another option for dealing with the problem of collisions is to use the carrier sense multiple access/collision detection (CSMA/CD) mechanism which is well known in the field of electrical bus networks common to a plurality of stations. Suffice to say that it provides in each station collision detection means, i.e. means for detecting situations in which, during the sending of a packet by the station, a concurrent signal is sent by another station. If a station detects a collision, sending of the current packet is interrupted. Subsequently, when the detection means indicate that no signal is being sent by any other station, a new attempt to send the same packet may be effected, after a certain inhibition time to ensure that each station has been able to reset itself.
The above principle can be transposed directly to optical networks comprising an optical bus, i.e. an optical link along which a plurality of stations that can send on the same wavelength are coupled.
To implement this method in the context of the tree optical network described above, it is necessary to adapt the links to route to each station each signal sent by each of the other stations.
This can be achieved in the manner shown in FIG. 2. For a network having N stations, the coupler C is an N to 3 star coupler (N =3 in the present example) having N input ports P1, P2, P3 connected to the respective fibers F1, F2, F3 leading to the stations and three output ports Q1, Q2, Q3. The output port Q1 is coupled to the end J of the common optical link OL and the other two output ports Q2, Q3 are coupled to each other through an isolator IS. Thus the combination S(1, 2, 3) of uplink signals S1, S2, S3 is liable to be sent simultaneously by the stations is re-injected into the fibers F1, F2, F3 in the downlink direction.
Stations implementing the signal detection and collision mechanism are known in the art. How a station STi (here i=1, 2, 3) can be implemented is nevertheless described in outline next with reference to FIG. 3.
The network access node consists of a coupling system executing the 1 to 3 coupling function, here simply represented by a coupler Ci. A first port of the coupler is coupled to the associated fiber Fi (here i=1, 2, 3) and constitutes the sending point Ki of the station. One of the other three ports opposite the first is connected to the receiver RX, the second to a collision detection system 3 and the third to the sender TX.
The receiver RX is able to detect the downlink wavelength λd, for example by means of a filter Bd that selects the wavelength λd followed by a photodetector PDd and electronic circuits (not shown in detail) that deliver the corresponding received data RD in electrical form.
The sender TX includes a sending system 4 for storing the data TD to be sent and converting it to optical form. The system 4 includes in particular a laser source tuned to the uplink wavelength λu and modulation means (not shown in detail). The sender TX further includes sending control means 5 adapted to command activation of the sending system 4 to send the uplink signals Si and to dialogue with the collision detection system 3.
The collision detection system 3 includes a receiver able to detect the uplink wavelength λu, for example by means of a filter Bu that selects the wavelength λu followed by a photodetector PDu. Taking into account the optical power level received at the uplink wavelength λu and the activity status of the sender TX (as indicated by a signal TR supplied by the sending control means 5), the system 3 can detect collisions, i.e. the presence at the sending point Ki of an optical wave at the uplink wavelength that did not come from the sender of the same station. If a collision occurs, the system 3 notifies the sending control means 5 of the sender (signal CD). In this case, a new attempt is made subsequently to send the data affected by the collision.
The collision detection system 3 also determines if there is any signal at the uplink wavelength λu propagating in the fiber in the downlink direction, and notifies the sending control means 5 of the sender accordingly (signal CS); this tells the sender whether it is authorized to send or not. The combination of the detection system 3 and the sending control means 5 thus constitutes a standard CSMA/CD type collision management system.
Compared to the method described above using time division multiplexing, the CSMA/CD solution has the advantage of being totally decentralized. However, by virtue of factors inherent to its theory, this technique offers poor performance in terms of the global traffic capacity of the network. Furthermore, the respective bandwidth requirements of the stations cannot be taken into account and the use of network resources is sub-optimal.